(Flux Method)
A flux method, which is one type of liquid-phase methods, can produce a gallium-nitride single crystal substrate having a low dislocation density. In the GaN single-crystal growth by the flux method, a seed crystal is generally used as a starting point to grow crystals of GaN. A substrate suitable for use in such growth is a wafer (seed crystal substrate) formed by growing a seed crystal layer of about 1 to 20 μm in thickness on a sapphire substrate by a vapor-phase method, such as a metal organic chemical vapor deposition (MOCVD) method and a hydride vapor phase epitaxy (HYPE) method (see Patent Document 1).
(MOCVD Method)
A metal organic chemical vapor deposition (MOCVD) method has been increasingly used to deposit a nitride semiconductor on a silicon substrate instead of the conventional sapphire substrate, thereby forming a power-device structure or a light-emitting diode (LED) element structure. The silicon substrate has a merit that a large-sized wafer for the silicon substrate is available.
(Composite Substrate)
In manufacturing semiconductor devices, such as a blue LED, a blue-violet semiconductor laser, and a power semiconductor, a composite substrate can be used that is formed by bonding an underlying substrate to a substrate formed from a group III nitride, such as gallium nitride (Patent Document 2).
In the power device or LED element using a gallium nitride thin film, generally, after a process for forming an element, to improve the heat dissipation properties, the silicon substrate or sapphire substrate is thinned by polishing and then bonded to an alumina substrate, an AlN polycrystalline substrate or the like having a higher thermal conductivity.